The effect of hypothermia on intracellular myocardial pH in humans is unknown. The acid-base status of blood during systemic hypothermia theoretically should have profound effects on myocardial tissue pH. Variations in myocardial tissue pH affect enzyme function and the ability of the organ to withstand ischemia. The purpose of this project is to determine whether myocardial pH varies with changes in blood pH. The NIH tissue pH probe has been inserted in the myocardium of 16 patients during hypothermic cardiopulmonary bypass. The acid-base scheme followed was either that of alpha-stat (N=3) or pH-stat (N=ll) and the tissue pH was compared to that of the myocardium using these two pH management schemes. Two patients had inadequate data. The preliminary results show that the changes in myocardial pH parallel those of the blood pH. The study is to continue.